A distant retrograde orbit (DRO), as most commonly conceived, is a spacecraft orbit around a moon that is highly stable because of its interactions with two Lagrange points (L1 and L2) of the planet–moon system.
In more general terms, an object of negligible mass can be in a DRO around the smaller body of any two-body system, such as planet–Sun or exoplanet–star.
Using the example of a spacecraft in a DRO around a moon, the craft would orbit in the direction opposite to the direction in which the moon orbits the planet. The orbit is "distant" in the sense that it passes above the Lagrange points, rather than being near the moon. If we consider more and more distant orbits, the synodic period (the period between two moments when the craft passes between the planet and the moon) gets longer and approaches that of the moon going around the planet, so that the sidereal period (the time it takes for the craft to come back to a given constellation as viewed from the moon) can become much longer than the orbital period of the moon. A hypothetical example with Europa has a sidereal period about eight times the orbital period of Europa.
DROs have been researched for several decades, but in 2022, NASA's Orion Spacecraft entered the orbit during the Artemis 1 mission and it was speculated that the Chang'e 5 orbiter may also have now done so.
The stability of a DRO is defined in mathematical terms as having very high Lyapunov stability, where an equilibrium orbit is "locally stable if all solutions which start near the point remain near that point for all time".
After dropping off return samples for Earth, China's Chang'e 5 (CE-5) orbiter first moved to Sun-Earth Lagrange point 1 (L1) in March 2021 for solar observations.In January 2022, CE-5 left L1 point for the lunar distant retrograde orbit (DRO) to conduct very-long-baseline interferometry tests in preparation for the next stage of China's lunar exploration program. According to The Space Review (TSR), this maneuver was depicted in Chinese government and academic documents. In February 2022, Multiple amateur satellite trackers observed CE-5 had entered DRO, which was the first spacecraft in history to utilize the orbit.
On 16 November 2022, the Space Launch System was launched from Complex 39B as part of the Artemis 1 mission carrying Orion to the moon.On 25 November it entered DRO and orbited the Moon in that orbit.
A distant retrograde orbit was one of the proposed orbits around Europa for the Jupiter Icy Moons Orbiter—principally for its projected stability and low-energy transfer characteristics—but that mission concept was cancelled in 2005.
A distant retrograde orbit was considered to be used for the proposed Asteroid Redirect Mission. Although the mission would end up getting cancelled, the research done with DRO in-mind, lead to the orbit being used for Artemis 1.
Two system requirements for the NASA Lunar Gateway, as published in the Baseline DSG-RQMT-001published in June 2019, mention the use of lunar DROs. Requirement L2-GW-0029, Single Orbit Transfer, states "the Gateway shall be capable of performing a single round trip transfer to Distant Retrograde Orbit (DRO) and back within 11 months". Requirement L2-GW-0026, Propulsion System Capability, states "the Gateway shall provide a fuel capacity that would support performing a minimum of two round-trip uncrewed low-energy cislunar orbit transfers between a near-rectilinear halo orbit (NRHO) and a distant retrograde orbit (DRO) and orbit maintenance for a period of 15 years between refueling". Although the selected orbit for the Gateway has been confirmed to be NRHO instead of DRO.
In the 2019 Daniel Suarez novel Delta-v, a 560-tonne crewed asteroid-mining ship Konstantin is constructed in a lunar DRO approximately 40,000 km (25,000 mi) above the Moon.
The Crew Exploration Vehicle (CEV) was a component of the U.S. NASA Vision for Space Exploration plan. A competition was held to design a spacecraft that could carry humans to the destinations envisioned by the plan. The winning design was the Orion spacecraft.
The National Aeronautics and Space Administration is an independent agency of the U.S. federal government responsible for the civil space program, aeronautics research, and space research. Established in 1958, NASA succeeding the National Advisory Committee for Aeronautics (NACA), to give the U.S. space development effort a distinctly civilian orientation, emphasizing peaceful applications in space science. NASA has since led most American space exploration, including Project Mercury, Project Gemini, the 1968–1972 Apollo Moon landing missions, the Skylab space station, and the Space Shuttle. NASA currently supports the International Space Station and oversees the development of the Orion spacecraft and the Space Launch System for the crewed lunar Artemis program, the Commercial Crew spacecraft, and the planned Lunar Gateway space station.
Artemis 1, officially Artemis I and formerly Exploration Mission-1 (EM-1), was an uncrewed Moon-orbiting mission. As the first major spaceflight of NASA's Artemis program, Artemis 1 marked the agency's return to lunar exploration after the conclusion of the Apollo program five decades earlier. It was the first integrated flight test of the Orion spacecraft and Space Launch System (SLS) rocket, and its main objective was to test the Orion spacecraft, especially its heat shield, in preparation for subsequent Artemis missions. These missions seek to reestablish a human presence on the Moon and demonstrate technologies and business approaches needed for future scientific studies, including exploration of Mars.
A subsatellite, also known as a submoon, or moonmoon, or a schmoon, is a "moon of a moon" or a hypothetical natural satellite that orbits the moon of a planet.
The Asteroid Redirect Mission (ARM), also known as the Asteroid Retrieval and Utilization (ARU) mission and the Asteroid Initiative, was a space mission proposed by NASA in 2013; the mission was later cancelled. The Asteroid Retrieval Robotic Mission (ARRM) spacecraft would rendezvous with a large near-Earth asteroid and use robotic arms with anchoring grippers to retrieve a 4-meter boulder from the asteroid.
Artemis 2 is the second scheduled mission of NASA's Artemis program and the first scheduled crewed mission of NASA's Orion spacecraft, currently planned to be launched by the Space Launch System (SLS) in November 2024. The crewed Orion spacecraft will perform a lunar flyby and return to Earth. Artemis 2 is planned to be the first crewed spacecraft to travel to the Moon, and beyond low Earth orbit, since the Apollo 17 mission in 1972.
Artemis 3 is planned as the first crewed Moon landing mission of the Artemis program and the first crewed flight of the Starship HLS lander. Artemis 3 is planned to be the second crewed Artemis mission and the first crewed lunar landing since Apollo 17 in December 1972. Originally scheduled to be launched in 2024, as of June 2023, the mission is likely not to take place before 2026.
The Lunar Gateway, or simply Gateway, is the first planned extraterrestrial space station. It will be placed in lunar orbit and is intended to serve as a solar-powered communication hub, science laboratory, and short-term habitation module for government-agency astronauts, as well as a holding area for rovers and other robots. It is a multinational collaborative project involving four of the International Space Station partner agencies: NASA, European Space Agency (ESA), Japan Aerospace Exploration Agency (JAXA), and Canadian Space Agency (CSA). It is planned to be both the first space station beyond low Earth orbit and the first space station to orbit the Moon.
EQUULEUS is a nanosatellite of the 6U CubeSat format that will measure the distribution of plasma that surrounds the Earth (plasmasphere) to help scientists understand the radiation environment in that region. It will also demonstrate low-thrust trajectory control techniques, such as multiple lunar flybys, within the Earth-Moon region using water steam as propellant. The spacecraft was designed and developed jointly by the Japan Aerospace Exploration Agency (JAXA) and the University of Tokyo.
A near-rectilinear halo orbit (NRHO) is a halo orbit with slightly curved – or nearly straight – sides between close passes with an orbiting body. The 2022 CAPSTONE mission is the first such orbit in cislunar space, and this Moon-centric orbit will serve as a staging area for future lunar missions. The orbit could be used with other bodies in the Solar System and beyond.
The Artemis program is a robotic and human Moon exploration program led by the United States' National Aeronautics and Space Administration (NASA) along with six major partner agencies— the European Space Agency (ESA), the Japan Aerospace Exploration Agency (JAXA), the Canadian Space Agency (CSA), the Israel Space Agency (ISA) and the Australian Space Agency (ASA), (ISRO) Indian Space Research Organisation. The Artemis program is intended to reestablish a human presence on the Moon for the first time since the Apollo 17 mission in 1972. The main parts of the program are the Space Launch System (SLS), the Orion spacecraft, the Lunar Gateway space station, and the commercial Human Landing Systems. The program's long-term goal is to establish a permanent base on the Moon to facilitate the feasibility of human missions to Mars.
Artemis 4 is the fourth planned mission of NASA's Artemis program. The mission will launch four astronauts on a Space Launch System rocket and an Orion to the Lunar Gateway and the second lunar landing of the Artemis program.
Artemis 5 is the fifth planned mission of NASA's Artemis program and the first crewed flight of the Blue Moon lander. The mission will launch four astronauts on a Space Launch System rocket and an Orion to the Lunar Gateway and will be the third lunar landing of the Artemis program. In addition, Artemis V will also deliver two new elements to the Gateway Space Station.
CAPSTONE is a lunar orbiter that will test and verify the calculated orbital stability planned for the Lunar Gateway space station. The spacecraft is a 12-unit CubeSat that will also test a navigation system that will measure its position relative to NASA's Lunar Reconnaissance Orbiter (LRO) without relying on ground stations. It was launched on 28 June 2022, arrived in lunar orbit on 14 November 2022, and was scheduled to orbit for six months. On 18 May 2023, it completed its primary mission to orbit in the near-rectilinear halo orbit for six months, but will stay on this orbit, continuing to perform experiments during an enhanced mission phase.
The Power and Propulsion Element (PPE), previously known as the Asteroid Redirect Vehicle propulsion system, is a planned solar electric ion propulsion module being developed by Maxar Technologies for NASA. It is one of the major components of the Gateway. The PPE will allow access to the entire lunar surface and a wide range of lunar orbits and double as a space tug for visiting craft.